Organic electroluminescent devices are self-emission type display devices and are expected for use in displays and lights. Organic electroluminescent displays have advantageous display performances such as higher visibility than conventional CRTs and LCDs, and no viewing angle dependency. Organic electroluminescent displays also have an advantage that they can be made lighter and thinner. Meanwhile, organic electroluminescent lights can be advantageously lighter and thinner and also, using a flexible substrate, organic electroluminescent lights may have a shape conventional lights cannot have.
An organic electroluminescent layer of an organic electroluminescent device has a multi-layered structure containing a light-emitting layer, a transparent electrode and other layers. Therefore, when light is emitted at an angle equal to or higher than a critical angle determined based on the refractive index of the organic electroluminescent layer and the refractive index of a medium into which the light is to be emitted, the light cannot be emitted to the air, totally reflected, and confined in the interior of the organic electroluminescent layer, so that the light is lost. According to calculation based on the classical Snell's law of refraction, when the refractive index n of the organic electroluminescent layer is 1.8 (the refractive index n of the organic electroluminescent layer is 1.7 to 1.85 according to NPL 1) and the distribution of light emitted from the organic electroluminescent layer is a Lambertian distribution, the light extraction efficiency of the light emitted to the air is about 30% due to the difference between the refractive index of the organic electroluminescent layer and the refractive index of the air. The remaining light accounting for about 70% cannot be emitted to the air since it is confined in the interior of the organic electroluminescent layer due to this difference in refractive index.
Solving the above problems to improve the light extraction efficiency requires preventing the above-described total reflection. In the process through which light emitted from the organic electroluminescent layer reaches the air, an angle of finally emitted light is determined by the difference between the refractive index of the organic electroluminescent layer and the refractive index of the air which is the lowest, so that the light extraction efficiency of the finally emitted light is determined. However, the totally reflected light in the organic electroluminescent device is energy-distributed by the refractive index of each layer of the organic electroluminescent device. For example, as illustrated in FIGS. 1A to 1D, the place where light is totally reflected when reaching air is varied with the relationship between the refractive index of each layer of the organic electroluminescent device and the refractive index of the organic electroluminescent device.
In the organic electroluminescent devices illustrated in FIGS. 1A to 1D, a first light-transmitting layer 105 is a transparent substrate, and a second light-transmitting layer 104 is a layer made of, for example, an adhesive and may be a protective layer.
As illustrated in FIG. 1A, no total reflection occurs between the organic electroluminescent layer 102 and the transparent electrode 103 since the refractive index (nel) of the organic electroluminescent layer 102 is 1.8 and the refractive index (n3) of the transparent electrode 103 (e.g., an ITO electrode) is about 2.0. Also, the refractive index (n1) of the first light-transmitting layer 105 and the refractive index (n2) of the second light-transmitting layer 104 are lower than the refractive index (nel) of the organic electroluminescent layer 102. When an outer layer is lower in refractive index than an inner layer in this manner, total reflection occurs at all the interfaces of the adjacent layers to the first light-transmitting layer 104 and the second light-transmitting layer 105.
Also, as illustrated in FIGS. 1B and 1C, when the refractive index of one of the first light-transmitting layer 105 and the second light-transmitting layer 104 is lower than the refractive index of the organic electroluminescent layer 102, the organic electroluminescent device has two or more interfaces where total reflection occurs.
Therefore, in order to improve light extraction efficiency in the organic electroluminescent devices illustrated in FIGS. 1A, 1B and 1C, a light-extracting unit has to be provided on each of the interfaces where total reflection occurs, and even providing a light-extracting unit only one interface of the interfaces where total reflection occurs is not enough to extract totally reflected light. Providing many light-extracting units is expected to require optimal design of light-extracting units and an organic electroluminescent device, which leads to complicated production process and cost elevation. When light-extracting units are provided on different places, the total efficiency of the light-extracting units is a product of the efficiencies of the light-extracting units. Meanwhile, since the light-extracting unit cannot extract all incident lights including totally reflected light and achieve 100% or 100% or more of the light extraction efficiency, the light extraction efficiency of the organic electroluminescent device is extracted to be lower as the number of the light-extracting units is greater. Also, in providing the light-extracting units, the interfaces where total reflection occurs newly arise unless consideration is made on the relative relationship between the refractive index of each light-extracting unit and the refractive index of the organic electroluminescent layer.
In FIG. 1D, when the refractive indices of the first light-transmitting layer 105 and the second light-transmitting layer 104 are higher than the refractive index of the organic electroluminescent layer 102, light emitted from the organic electroluminescent layer 102 all enters the first light-transmitting layer 105 adjacent to the air. Thus, its light extraction efficiency can be improved by providing a light-extracting unit between the first light-transmitting layer and the air. Also, by optimizing the light-extracting unit, high light extraction efficiency is expected to be obtained.
Furthermore, in order to avoid total reflection of light due to the difference in refractive index between the organic electroluminescent layer and the air to improve light extraction efficiency, there have been proposed various light-extracting units such as a lens, a lens array, a prism, a prism array, a fine concavo-convex structure and a fine particle layer. However, these improvements in the light-extracting unit do not have sufficient effects of improving the light extraction efficiency.
FIGS. 2A to 2D each illustrate that light emitted from the organic electroluminescent layer enters the light-extracting unit and is extracted to the outside. Here, the refractive index of the organic electroluminescent layer is assumed to be equal to or higher than the refractive index of the light-extracting unit.
FIG. 2A illustrates a case where the light-extracting unit is a fine concavo-convex structure 106. In this case, among light emitted from the organic electroluminescent layer 102, light that is originally able to reach the air is converted at a certain rate to light that is unable to reach the air as well as light that is originally unable to reach the air is converted to light that is able to reach the air, depending on diffraction efficiencies based on the structure of the fine concavo-convex structure 106 and the wavelength and angle of the light entering the fine concavo-convex structure. The light that is still unable to reach the air even after conversion returns to the interior of the organic electroluminescent layer, hits the reflective electrode and re-enters the fine concavo-convex structure. Nevertheless, some light components are lost due to the finite size of the organic electroluminescent layer and to the absorption into, for example, each organic electroluminescent layer and the reflective electrode.
FIG. 2B illustrates a case where the light-extracting unit is a lens (lens array) 107. In this case, light entering the lens is converted in angle with the lens structure at a certain probability and as a result, some light components travel toward the air and other light components are reflected toward the organic electroluminescent device. In the case of a hemispherical lens, for example, almost all of the light components passing from the organic electroluminescent layer 102 through the center of the lens 107 or its vicinity can reach the air regardless of their angles. Meanwhile, the light components passing the edges of the lens or its vicinity are confined in the lens, reflected on the reflective electrode of the organic electroluminescent device, and re-enter the lens 107, so that some of them are emitted to the air. Although the lens allows more light components to be emitted to the air per one time as a result of avoidance of total reflection than in the fine concavo-convex structure, the lens has a similar problem of absorption to the fine concavo-convex structure and thus is unable to achieve an efficiency of 100%. Also, the light extraction efficiency depends on the structure of the lens and the light distribution of incident light.
FIG. 2C illustrates a case where the light-extracting unit is a prism (prism array) 108. In this case, light emitted from the organic electroluminescent layer 102 enters the prism 108 and is converted in light emission angle with characteristics such as an oblique angle of the slope of the prism and an apex angle of the prism. As a result, some light components reach the air and other light components return to the organic electroluminescent device. Similar to the case of the lens, there are light components having angles at which they cannot be emitted from the prism, and these light components are reflected on the reflective electrode of the organic electroluminescent device. Every time when light components that are not absorbed re-enters the prism 108, these light components are emitted to air at a certain rate, but a light extraction efficiency of 100% cannot be obtained similarly.
FIG. 2D illustrates a case where the light-extracting unit is a fine particle layer 109. In the fine particle layer 109, the fine particles scatter light entering the polymer due to the difference in refractive index between the fine particles and the polymer, determining scattering pattern and scattering efficiency of the light in accordance with the light distribution of the incident light and characteristics of the fine particles and the polymer. As a result, to-be-totally-reflected light that is originally unable to reach the air is converted to light that is able to reach the air. The light that is still unable to reach the air even after conversion returns to the interior of the organic electroluminescent layer 102, is reflected on the reflective electrode, and re-enters the fine particle layer, where it is scattered and emitted to the air at a certain scattering efficiency. However, it is inevitable to lose light similarly.
As discussed above, only use of various light-extracting units such as a lens, a lens array, a prism, a prism array, a fine concavo-convex structure and a fine particle layer cannot make sufficient improvement in light extraction efficiency. As is clear from the above discussion, the light distribution of light entering the light-extracting unit also influences the light extraction efficiency of the organic electroluminescent device. To speak of extremes, when energy of light entering the light-extracting unit concentrates on an optimal angle of the light-extracting unit, the light-extracting unit shows the maximum light extraction efficiency. Needless to say, although light emitted from the organic electroluminescent device cannot be at an extreme distribution, it is necessary to optimize the light distribution of light entering the light-extracting unit.
Meanwhile, analysis of interaction between light and the lens, lens array, prism, prism array, fine concavo-convex structure or fine particle layer indicates that when light is emitted from the organic electroluminescent layer to enter these structures and then is re-emitted from them, the light distribution of the light emitted to another medium is difference from the light distribution of the incident light; i.e., the original light distribution is converted with these structures. These structures can be used also as a light distribution-converting unit.
Here, care should be taken about the difference from another light distribution-converting unit in the fine particle layer. Specifically, since the fine concavo-convex structure, lens and prism use the difference in refractive index at the interfaces, the refractive index of a medium at the light-incident side cannot be equal to the refractive index of a medium at the light-emitted side. Also, in order to simply convert the light distribution to avoid total reflection, the refractive index at the light-incident side of the fine concavo-convex structure, lens, prism, etc. has to be equal to or higher than the refractive index of the organic electroluminescent layer and the refractive index of a medium at the light-emitted side has to be higher than the refractive index of a medium at the light-incident side.
The fine particle layer uses the difference in refractive index between the fine particles and the polymer and thus does not have the above limitations. The refractive index of the polymer at the light-emitted side may be equal to the refractive index of a medium at the light-incident side and the refractive index of the fine particles may be higher than the refractive index of the polymer. Therefore, there is no particular need to use a material having a higher refractive index than the organic electroluminescent layer. In addition, fine particles can be selected from a wide variety of fine particles, which gives a large degree of freedom to realization.
As already illustrated in FIGS. 2A to 2D, properly converting the light distribution of light emitted from the organic electroluminescent layer can lead to further improvement in light extraction efficiency. Preferably, the fine particle layer is used as a light distribution-converting member for light emitted from the organic electroluminescent layer, and fine structures such as a prism are used as a light-extracting member. Combining them can lead to further improvement in light extraction efficiency.
Any of the light-extracting units generates light that is changed in emission angle and returned to the interior of the organic electroluminescent device. In order to extract such light, the reflective electrode of the organic electroluminescent device is used to reflect and re-enter the light to the light-extracting unit. The reflectivity of the reflective electrode of the organic electroluminescent device greatly influences light extraction, which requires a reflective electrode having a high reflectivity relative to the organic electroluminescent layer.
Here, relevant prior art references disclose organic electroluminescent devices where a total reflection interface exists only at the interface with air considering the layer structure and the layer material, although PTLs 1 to 3 do not specifically describe them. PTLs 1 to 3 exemplify a fine particle layer, a lens and a prism as light-extracting units, but do not study at all synergistic effects for light extraction efficiency obtained combining a light distribution-converting unit or characteristics of each unit, especially scattering by fine particles (serving as a light distribution-converting unit), with another unit in order to obtain sufficient light extraction effects. In addition, studies on reflective electrode are not enough.
PTL 4 proposes an organic electroluminescent device where a volume diffuser (fine particle layer) and a micro-structure (prism layer) are combined together. In this proposal, however, the fine particle layer and the prism layer are not adjacent to each other, and both of the layers are used as light-extracting units. Therefore, the organic electroluminescent device has a problem that the light extraction efficiency is low.
As described above, at present, there has not been provided an organic electroluminescent device that is excellent in external extraction efficiency of emitted light and able to attain reduced power consumption and prolonged service life.